Wood membrane hygrometer



J. w. CASE 2,728,228 WOOD MEMBRANE HYGROMETER Original Filed Dec. 13,1951 Dec. 27, 1955 2 Sheets-Sheet l INVENTOR ATTORNEYS Dec. 27, 1955 J.w. CASE WOOD MEMBRANE HYGROMETER 2 Sheets-Sheet 2 Original Filed Dec.13, 1951 ml I h FIG. 5

RELATIVE HUMIDITY INVENTOR JAMES W. CASE ATTORNEYX WOOD MERANEHYGROMETER James W. Case, Fairfax, Va.

Original application December 13, 1951, Serial No. 261,570, new PatentNo. 2,682,858, datedluly 6, 1954. ll sivi tllcg and this application May4, 1953, Serial No. 3 3,

5 Claims. (Cl. 73-337) (Granted under Title 35, U. S. Code (1952), sec.266) This invention relates to improvements in hygrometers, and moreparticularly relates to devices for determining the relative amount ofmoisture in the atmosphere by meansof thin wood membrane sensitiveelements.

This application is a division of copending application Serial No.261,570 filed December 13, 1951, now U. S. Patent No. 2,682,858, issuedJuly 6, 1954.

The purpose of this invention is to provide an inexpensive, accurate,rugged instrument which can be relied upon for visual indication of therelative humidity inside of packages, containers, shipping cases,igloos, buildings, etc., in which perishable material is shipped orstored in a dehumidified atmosphere as a means of preservation.

The technique of preserving perishable equipment by means ofdehumidified air was developed during and succeeding World War II, whenit was found by numerous investigators that rusting and corrosion ofmetals is greatly reduced in an atmosphere below 50% relative humidity;that positive protection against rust and corrosion is obtained in airbelow 30% relative humidity and that most forms of mildew are preventedfrom growing below 70% relative humidity. These discoveries led to thedevelopment of standardized procedures for dry air preservation whichare being widely practiced by the military services and commercialcontractors.

Essentially, the procedure consists of enclosing the material orequipment being preserved in a moisture vapor barrier with silica-gel,which takes'the moisture out of the enclosed air space,-either by staticadsorption or by stripping the moisture from the air which is circulatedthrough the silica-gel bed bytmechanical means.

Since the moisture vapor barriers of such enclosures permit a continualsmall leakage of moisture from the outer atmosphere into the enclosures,some means of occasionally checking the moisture quality of the enclosedatmosphere is a necessary adjunct to this preservation process, therebyto inform maintenance and inspection personnel whether or not theatmosphere of the enclosure is below the safe upper critical limit ofrelativehumidity and assure adequate protection of the material underpreservation.

Presently available means for the indication of the relative humidity indehumidified shipping packages and containers are inadequate becausethey are either too costly and cumbersome, or because they are notaccurate within the critical values of moisture which must not beexceeded.

One conventional means utilizes cobalt chloride impregnated silica-gelto indicate the moisture condition of the container atmosphere bycertain characteristic colors, depending upon the atmospheric relativehumidity at which the silica-gel is in moisture equilibrium. Thisindicating means is not only limited to a qualitative indicacation ofthe moisture in the air, but is also subject to false indication, due toits gradual accumulation of moisture with each daily peak ofrelative-humidity (caused by atmospheric temperature change in thecontainer). This false indication results in unnecessary expense in theStates Patent lice opening of the enclosure to inspect the contents,renew the silica-gel charges, etc.

Another available means employs mechanical hygrometers having sensitiveelements of human hair, "animal membrane, paper-metal laminate, etc.While such instruments give considerably more accurate indication of therelative humidity than the silica-gel indicators above described, theyhave not been generally adopted for use in shipping and storagepackages, because they are too expensive in comparison with the value ofmaterial in the individual containers or packages; lack the desirablecompactness for adaptation; aresubject to accumulative scale errors dueto aging efiects on their sensitive elements and are in general tootrail to withstand the shock and vibration to which packages andcontainerare subjected during shipment.

The present invention contemplates the use of the hygroscopicity of woodand more particularly, the distortional characteristics of thin woodmembranes in moisture equilibrium with the atmosphere as moisturesensitive elements in low-cost, compact, reliable hygrometers, humidityindicating devices, controllers, etc.

The constants of the characteristic deformation of various species ofwood with changes of moisture content have been well established duringthe practice of kiln drying of lumber by the lumber industry and inresearch carried out by the Forest Products Laboratory of the U. S.Department of Agriculture To quote the latter authority:

Wood shrinks most in the direction oi the annual growth rings(tangentially), about one-half as much as this across these rings(radially) and very little, as a rule, along the grain (longitudinally?The shrinkage that occurs when the moisture content is reduced below thefiber-saturation point is basically dependent upon the amount ofmoisture removed from the cell walls. For practical purposes, swellingmay be considered the reverse of shrinking. For purposes of calculatingshrinkage, or swelling, the moisture content-shrinkage relationship maybe considered a direct one.

For reasons which are explained later, the selection of the radialshrinkage and swelling of wood are characteristics which can'bedependent upon for accurate indication of the amount of moisture in theatmosphere bymeans of thin wood membranes. Further, there have beenselected species of wood' which are durable; which are easily worked andwhich have suificient moisture-distortion gradients for actuationofindicating' means in mechanical hygrometers. For example, theu'nitradial'distortion of threetypic'al species of wood, overthe full rangeof relative humidity are calculated from Forest Products Laboratory dataand are indicated in the following table.

Unit radial distortion of wood in moisture equilibrium at variousrelative humidities and F. (DB) Length-Inches RH hjlolstturte on enEastern Percent Yellow vglggc Poplar Hickory 31 1. 000 1.000 1.000 20. 89925 987 976 16 988 .98 964 13 986 .977 9575 10. 9 984 974 9525 9. l 9B3972 948 7. 6 982 969 944 6 981 9675 940 4. 5 98D 966 937 2. 5 979 .963'9325 It has been found that the moisture distortion characteristics ofwood, ab'ove cited, are not useful for the moving force in amechanicalhygrom'e'ter in a thick section cut from the tree,as in a piece oflumber, but the sensitive element mustbe made in a very thin membranewhich can rapidly respond to changes of the atmospheric moisture.According to the literature, wood was one of the first materialsutilized in the construction of hygrometer sensitive elements. However,all types of wood sensitive elements employed by previous inventors areof relatively thick sections, in which an appreciable time is requiredfor equal wetting of the mass and swelling of the internal fibers. Thiscircumstance causes objectionable time lag between changes ofatmospheric moisture and the desired dimensional change of the sensitiveelement. The inherent time lag and sluggish nature of wood sensitiveelements employed by previous inventors has generally prevented the usedof this type of element in commercial hygrometers which normally employhuman hair, animal membrane, paper-metal laminate, etc. as sensitiveelements. It has been found, further, that all such organic fibers havean inherent time lag of at least 40 minutes between changes of relativehumidity and that, for comparable performance, a wood fiber must be ofapproximately the same thin section, or at least, as thin as practicablewithin the limits of the mechanical factors incident to the service ofthe instrument.

There has therefore been produced a wood hygrometer sensitive element ofcomparable thinness, accuracy and sensitivity to other organic fiberswhich are employed in hygrometers.

From these statements it will be understood that one of the objects ofthe invention is to provide an improved moisture indicating instrumentfor the guidance of shippers and inspection personnel who are chargedwith the safe transit or storage of valuable equipment which is subjectto rusting, corrosion and mildew and which is preserved by means ofmoisture vapor barriers and dry air.

Another object of the invention is to provide a moisture indicatingdevice which is contrived to give warning, by means of a visualmechanical signal, when the critical value of relative humidity indehumidified shipping packages or containers is reached.

Another object of the invention is to provide moisture sensitiveelements of thin wood membranes for hygrometers and hygrosignals.

Another object of the invention is to provide moisture sensitiveelements for hygrometers and hygrosignals which are resistant to theeffects of solvent vapors, such as methyl-ethyl-ketone, ethyl alcohol,and petroleum solvent which are commonly used in moisture barriers andin metal preservatives.

Another object of the invention is to provide moisture sensitiveelements for hygrometers which will have uniform moisture-distortioncharacteristics when produced in quantity, so that the hygrometers inwhich they are used may have duplicate scales and will assure acceptibleaccuracy over the full range of relative humidity.

Another object of the invention is to provide simplified mechanicalmagnification means for hygrometer sensitive elements to eliminateaccumulative instrument errors due to strain on the elements and shockor vibration during service. I

Another object of the invention is to provide moisture sensitiveelements which are responsive with a minimum of lag to the fluctuatingmoisture vapor conditions in the atmosphere.

Another object of the invention is to utilize the steps of a method ofproducing the critical membraneous-wood tube which is the sensitiveelement of the hygrometer.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawings wherein:

Fig. l is a perspective view illustrating the first me- I chanical stepof the method of producing the sensitive element;

Fig. 1A is a diagrammatic perspective view showing the method of cuttinga block from a log preparatory to obtaining a wood membrane which isexpansive and contractive radially in the tree;

Fig. 2 is a plan view of the membraneous-wood shaving, stretched outpreparatory to cutting;

Fig. 3 is a perspective view of a tube wherein the membraneous woodsections are joined for rectilinear expansion and contraction;

Fig. 4 is a diagram illustrating a development of a spiral winding ofthe wood membrane for rotational expansion and contraction;

Fig. 5 is an elevation of a typical wood membrane hygrometer wherein thesensitive element is rectilinearly expansive and contractive;

Fig. 6 is a perspective view of a modification of the hygrometer whereinrectilinear dimensional changes of the sensitive element are changedinto rotary motion, parts being broken away, and

Fig. 7 is a perspective view of a modification of the hygrometerutilizing a spirally wound wood membrane for the sensitive element.

The method of making the sensitive element is described first, then itsapplications. In Fig. 1, 10 designates a block of close-grained, wellseasoned soft wood. Species that are regarded as suitable for theinstant purpose include white pine (Pinus strobus), yellow poplar(Liriodendron tulipifera) and red cedar (.hmiperus virginiana). Theblock 10, Fig. l, is cut from a log 9, Fig. 1A, according to the commoncommercial practice of quarter sawing to bring out the radial grain ofthe wood. As shown in Fig. 1A, the block 10 is cut from the timber sothat the faces 10a, 10b of the block are purposely in parallelorientation with the radial plane a, a, b, b. Now when a shaving is cutfrom the face 10b, Fig. l, in the direction of the grain and across thering growth laminations, there is produced a thin membrane 11, Fig. 2,which utilizes only the radially distortional constants of the woodalong the line A-B, Fig. 1. This manner of cutting the block insures auniformity of distortion in the active element of the hygrometer whichcould not be obtained in any other known way. It is this uniformity ofdistortion which is responsible for the manner of response of thehygrometer element to ambient moisture as set out herein. The desiredspecimen having been selected, a shaving 11 is cut from one side in themanner described above.

A sharp carpenters plane is used for this purpose, or any equivalentinstrument that will produce a uniform, unbroken shaving from .0035 ofan inch to .005 of an inch thick. The width of the block, hence of theshaving, is about 1% inches. Much wider shavings could be produced withproper tools. This would avoid the necessity for piecing as below, andwould make possible a sensitive element in one piece. The shaving thusproduced is quite flimsy and has but little strength across its width.Consequently, care has to be taken in the next step of smoothing theshaving out as in Fig. 2 and cutting from it sections 12 of uniformlength. The cuts are made along the lines 13.

These sections 12 are assembled on a mandrel (not shown) and united atthe joints 14 with an alkyd resin, cellulose nitrate, or phenoliccement, available commercially. The longitudinal joints are desirablystaggered around the mandrel. The result is the cylinder 16 in which thejoints furnish reinforcement to the otherwise frail sections. Thiscylinder is the sensitive element. Now, since the sections 12 areassembled end on end with their axes corresponding, in the tree, to theradial plane a, a, b, b, Fig. 1A and the line A B, Fig. 1, the cylinder16, Fig. 3, forms a moisture-sensitive element which is responsive onlyto the radially expansive constant of-the tree wood. When the humidityis on the increase the cylinder will expand lengthwise and as themoisture content diminishes the cylinder will contract. 7 Tests to dateshow that a maximum dimensional change of an elementas shown in Fig. 3when :made of white pine is on the order of .002 inch per inch length.

Having made the sensitive element according to the foregoing method, thecylinder '16 is embodied 'in the typical wood membrane hygrometer'shownin Fig. 5. Any desired kind of supporting -panel.17 is used for themount. A mount composed of a metal piece-18 is desirably but notnecessarily riveted on the panel. This piece carries the stationarypivot pin 19 which by means of the adapter 20 allows the cylinder 16 toswing on a slight arc. The cylinder 16 has its lower end secured to theadapter 20 by means of the same air-drying cement used for fabricationof the cylinder 16 of sections 12.

A similarly shaped and secured adapter 22 at the upper end of thecylinder makes connection at the movable pivot 23 with the short arm ofan indicating needle 24. The adapters assist in preserving the tubularform of the sensitive element and augment the reinforcement of themembrane. This needle has a stationary pivot 25, and its free endtraverses a scale 26 which is graduated in percentages of relativehumidity. The needle setting illustrated in Fig. indicates a 100%absorption of moisture by the cylinder 16. As the cylinder losesmoisture it shrinks axially, whereupon the needle swings clockwise andindicates a diminishing moisture content both of the cylinder and theambient air.

The form of hygrometer in Fig. 6 primarily is for use on a desk or thelike. It has a base 62 from which a panel 9 63 is rigidly erected. Theindicating needle 65 plays over the graduations 66 which are numbered inpercentages of humidity. The pivot spindle 67 of the needle has bearingin a support bracket 68 which is fastened to the back of the panel andhas a part 69 that extends downward and inside of the upper end of thesensitive element 16.

This element 16 still is of the type in Fig. 3, and one end of it iscemented to the base as at 71. A thin but strong crossbar 72, preferablybut not necessarily of hardwood, is cemented at its ends to the element16 and in bridging position across the upper open end. This crossbarraises and lowers with the axial expansion and contraction of theelement 16, and these motions are refiected in the needle 65 through aFiberglas thread 73 to which the crossbar is aflixed by an adhesivesuch, for example as cellulose nitrate,. alkyd resin or phenolic cement.

The thread 73 is a loop, being made so, conceivably, by connecting theends by the same spot of cement that aflixes the loop to the crossbar.The thread is stretched rather tightly over upper and lower sheaves 74,75, respectively on the spindle 67 and on a pin 77 that spans thedistance between the extremities of the part 69. The sheave 75 is anidler. Being composed of Fiberglas hence non-hygroscopic in character,the thread 73 will remain as tight as it is originally stretched sinceit will not respond to the humidity changes to which the element isexpected to respond.

It is readily seen in Fig. 6 that the linear dimensional changes of thesensitive element 16 will be converted into circular motion at thespindle 67 and needle 65. The choice of the right sheave diameter andindicating needle length will produce the desired scale deflection.

Fig. 4 illustrates the first and only modification of the sensitiveelement itself. Here, instead of being made in sectional form theshaving in Fig. 1 is left intact and is formed into a unitary woodshaving 78, spirally wound into a tubular element and made to remain soby spots of cement at intervals. The purpose is accomplished, forexample, by winding the shaving spirally on a bi mandrel and lightlycementing the edges at points about V2" apart with cellulose nitrate,alkyd resin or phenolic cement.

The object of this is to produce a non-rigid tube which will permitrespective rotation between the two opposite ends, due to spiraldistortion upon the accretion and loss of moisture. It has been foundthat such a tube 6" long'will rotate approximately '30" between oppositeends from' a'mbient conditions of between 50% and 100% relativehumidity.

In further explanation of Fig. 4, the rotative effort caused by theexpansion (or contraction) of the wood membrane 'will be understood fromthe development of the spirally wound element. In accordance with thetransverse grain expansion set "out before, the maximum dimensionalchange of the developed sheet is diagonally between points b and d. Thisrepresents a distortion of the rectangular sheet into a parallelogram asshown at a, b, c, d. With points 0, d fixed, as they will be by thecementing down of the tube, the movement of points a, b to a, bcorresponds to clockwise motion at the free end of the tube, in otherwords to an extent where the element is in equilibrium with theparticular moisture content of the ambient air.

Fig. 7 illustrates the utilization of the element in Fig. 4. Aninstrument case 79 of any chosen type forms the housing for the element16 and the mount for the spindle 80 of the needle 82. One end of thetube (0, d in Fig. 4) is cemented at 83 to one wall of the case. A holein the opposite wall forms a bearing for the spindle. The inner end ofthis spindle is affixed to a plug 84 in the otherwise open end of thetube. The needle 82 is aflixed to the outer end of the spindle and, asin the other instances, plays over a set of humidity graduations 85.

The operation of the various forms of the hygrometer is generalizedthus: The accretion or loss of moisture by the wood-membrane sensitiveelement 16 causes a dimensional change therein which is utilized toshift the indicating needle. This occurs either by axial elongation orcontraction or by the twisting of the tubular body. Under the firstsetup the humidity indicator needle is moved either by a sheave system(Fig. 6) or by a system of levers and pivots (Fig. 5). Under the secondsetup the indicator turns in conformity to the twisting of the sensitiveelement (Fig. 7).

The invention described herein may be manufactured and used by and forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

What is claimed is:

1. In a hygrometer, a mounting means and a relatively movable means saidmeans being located a distance apart from each other, and a moisturesensitive element spanning said distance being connected at one end tothe mounting means and having its other end in operative proximity tothe movable means, said element consisting of a wood membrane having awall thickness up to fivethousandths of an inch and being made intubular form for requisite mechanical strength, said tube having thewood grain extending spirally thereof.

2. In a hyrogrometer, a moisture sensitive element consisting of atubular membrane fixed at one end and movable at the other end due tochanges in its moisture content, a spindle mounted to turn according tosaid changes and having a needle to indicate percentages of humidity bymovement in respect to a reference point, an idler in establishedrelation to the spindle, and a loop of nonhygroscopic thread embracingthe idler and spindle and connected to the movable end of the sensitiveelement.

3. In a hygrometer, a support, a moisture sensitive element consistingof a tubular membrane fixed on the support at one end and bodilyrectilinearly movable for displacement of its other and free end due tochanges in its moisture content, a bracket attached to the support andcarrying an idler sheave in proximity to the free end, a spindlejournaled on the bracket and having an indicating needle, a loop ofnon-hygroscopic thread embracing the spindle and idler, and means bywhich the thread is fixedly connected at one place in its length to thefree end of the sensitive element.

References Cited in the file of this patent UNITED STATES PATENTS780,791 Gerrer Jan. 24, 1905 1,293,527 Ovington Feb. 4, 1919 7 1,336,380Sampietro Apr. 6, 1920 1,675,302 Roemer June 26, 1928 FOREIGN PATENTS558,089 Germany Sept. 2, 1937 OTHER REFERENCES Abstract application No.742,997, C. F. Wallace, filed 10 Apr. 22, 1947, vol. 645, p. 1027, O. G.Apr. 17, 1951.

